Several methods have been described in the literature for producing demineralized or low-ash coal for fuel and other industrial applications, but none have achieved sustained commercial use.
A process was developed in Germany during the 1940's for removing ash-forming mineral matter from physically cleaned black coal concentrates, involving heating the coal as a paste with aqueous alkali solution, followed by solid/liquid separation, acid washing and water washing steps. Reports on this process detail a practical chemical demineralizing method. German practice showed that a demineralized coal with an ash yield of 0.28% could be produced from a physically cleaned feed coal which had an initial ash yield of 0.8%.
The coal-alkali feed paste was stirred at 40°-50° C. for 30 minutes, then pumped through a heat exchanger to a continuously operable gas-heated tubular reactor in which the paste was exposed to a temperature of 250° C. for 20 minutes, under a pressure of 100-200 atmospheres (10-20 MPa). The reaction mixture was then passed through the heat exchanger previously mentioned, in order to transfer heat to the incoming feed, then cooled further in a water-cooled heat exchanger.
The cooled paste was diluted with softened water, then centrifuged to separate and recover the alkaline solution and the alkalized coal. The latter was dispersed to 5% hydrochloric acid, then centrifuged to recover the acidified coal and spent acid and redispersed in water. The coal was filtered from this slurry, dispersed again in another lot of water and centrifuged to recover the resulting low-ash coal as a damp solid product.
American and Indian researchers used broadly similar chemical methods, with variations in processing details, to produce low-ash coals from other feed coals, most of which had much higher starting ash levels than the coals than the Germans used. Another American group (at Battelle) claimed advantages for:                (a) Mixed alkali leachants containing cations from at least one element from Group IA and at least one element from Group IIA of the Periodic Table;        (b) Filtration or centrifugation of the alkalized coal from the spent alkaline leachant, either at the reaction temperature or after rapid cooling to less than 100° C., in order to minimise the formation of undesired constituents, presumably sodalite or similar compounds;        (c) Application of the process to low-rank coals which dissolve in the alkali and which can be reprecipitated at a different pH from the mineral matter, thus allowing separation and selective recovery.        
Other researchers had studied scientific aspects of alkaline extraction of sulphur and minerals, including the relative merits of different alkalis. Most American work has been directed at the removal of sulphur rather than metallic elements, and the acid treatment step is often omitted. However, an American group (at Alcoa) has chemically cleaned coal to less than 0.1% ash yield, concurrently achieving large reductions and low final concentrations of iron, silicon, aluminium, titanium, sodium and calcium. The aim was to produce very pure coal suitable for conversion into electrode carbon for the aluminium industry. This was achieved by leaching powdered coal with hot aqueous alkaline solution under pressure (up to 300° C.), then successively with aqueous sulphuric acid and aqueous nitric acid at 70°-95° C.
Australian patent no. 592640 (and corresponding U.S. Pat. No. 4,936,045) describes a process for the preparation of demineralized coal. This process includes the following steps:                (a) forming a slurry of coal particles, preferably at least 50% by weight of which particles have a maximum dimension of at least 0.5 mm, with an aqueous solution of an alkali, which solution has an alkali content of from 5 to 30% by weight, such that the slurry has an alkali solution to coal ratio on a weight basis of at least 1:1;        (b) maintaining the slurry at a temperature of from 150° to 300° C., preferably 170° C. to 230° C., for a period of from 2 to 20 minutes substantially under autogenous hydrothermal pressure and rapidly cooling the slurry to a temperature of less than 100° C.;        (c) separating the slurry into alkalized coal and a spent alkali leachant solution;        (d) regenerating the alkali leachant solution for reuse in step (a) above by the addition of calcium or magnesium oxide or hydroxide thereto to precipitate minerals therefrom;        (e) acidifying the alkalized coal by treatment with an aqueous solution of sulphuric or sulphurous acid to yield a slurry having a pH of from 0.5 to 1.5 and a conductivity of from 10,000 to 100,000 μs;        (f) separating the slurry into acidified coal and a spent acid and a spent acid leachant solution; and        (g) washing the acidified coal.        
Although the process described in Australian patent no. 592640 can produce a demineralized coal product having on ash content of less than 1% by weight and as low as 0.50% by weight, significant opportunities arise if the ash content can be reduced to even lower levels. If the ash level can be reduced to levels even lower than that achieved in Australian patent no. 592640, the demineralized coal product may be used as a fuel directly fired into a gas turbine. In this use, the demineralized coal could replace natural gas as a fuel for the gas turbine. Such demineralized coal could also be used as an alternative to heavy fuel oils and as a high purity carbon source for the production of metallurgical recarbonisers, carbon electrodes for aluminium production and alternative reductants for high purity silicon manufacture. The contents of U.S. Pat. No. 4,936,045 are herein incorporated by cross-reference.